COS 2-6 - Land use change, nutrient pollution, and sedimentation seasonality in a South Pacific coral reef island system

Abstract: Increasingly severe coral mortality events are attributed to rising ocean temperatures, heightened cyclone intensity, and increased sediment and nutrient pollution along tropical coastlines. Land use and land use change modify sediment and nutrient pollution on reefs by altering the transport of terrestrial particulate matter into lagoons via water runoff. Therefore, seasonal variation in rainfall may interact with land use in driving patterns of coral mortality at the local scale. However, few empirical studies explore this interaction, leaving uncertainty about how seasonal abiotic environmental variation may modify anthropogenic coral stressors.

Here, we report on a 40-year land use change study in the ridge-to-reef system of Moorea, French Polynesia. We used field data and photointerpretation to generate a high-resolution land cover time series with the goal of informing hydrodynamic models that explain spatiotemporal variation in island-wide water runoff regimes. We tracked dominant land cover conversions and show that forests are vanishing more rapidly than other land cover types, while agricultural and developed areas are expanding. We also monitored water column temperature and nutrient concentrations in an island-wide network of lagoon sampling sites, revealing spatial patterns in the magnitude and variability of nutrient loading and heat stress. We then connect land cover at the watershed scale to spatiotemporal variation in lagoon temperature and nutrient regimes, identifying pathways through which coral bleaching and mortality may relate to terrestrial change. Because cleared land is associated with increased particulate matter in runoff, the duration of development projects – and consequently the amount of time a developing land parcel remains clear – may be an important determinant of lagoon sedimentation and nutrient pollution. Therefore, planning development projects and other land clearing operations around the rainy season may help mitigate some of these effects. Future development plans in coral reef systems should account for land use impacts on terrestrial runoff, and incorporate mitigation efforts such as riparian buffers. Simulation models combining terrain, land cover, and precipitation data will inform how seasonality in precipitation drives variation in total sedimentation and nutrient pollution in lagoons, shedding light on downstream effects of land use on coral reef ecosystems in the South Pacific.